Along with rapid industrial development, the amount of coal ash generated as a by-product in thermal power plants has also increased as the use of electric power has increased in recent years. The amount of coal ash generated was approximately 4,500,000 tons in Korea in 2000 and it is expected that approximately 6,000,000 tons of coal ash will be generated in the year 2010.
Generally, the term “coal ash” refers to powdery substance solids which are left as residues after the combustion or the incineration of coal. Most coal ash is generated in the thermal power plants. Besides these power plants, waste incinerators, combined heat and power plants (CHP), and other industrial fields generate coal ash in the course of the combustion process. Since the coal ash is a residue from combusted coal, it is a material that is recyclable as inorganic substances, for example, SiO2, Al2O3 and Fe2O3. However, the coal ash is subjected to a combustion process, and hence it always contains unburnt carbon additionally, which causes a problem in applying technically.
The coal ash is largely classified into fly ash, cinder ash and bottom ash depending on its generation site. Among these, the bottom ash is ash that collects at the bottom of the combustion chamber, which is also called “clinker ash”. The bottom ash occupies about 10-15% of the total amount of coal ash generated.
The bottom ash is produced by allowing granules formed by means of sintering within a combustion furnace to fall to the bottom of a boiler to form solids and the solids to be then pulverized to a particle size of less than 25 mm using a pulverizer. Generally, the bottom ash produced by pulverization of the pulverizer has a particle diameter of 1-20 mm or so, and comprises SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O and K2O as main chemical ingredients. The bottom ash particles form an aggregate of very irregular particles including particles which have a porous surface and remains unburnt upon the combustion of coal ash, and particles which resemble sand particles and have a diameter of more than 5 mm. In addition, it is known that the bottom ash has various particle shapes or chemical components depending on ash generation sites, has a specific gravity of 2.1-2.7 or so, has a dry weight of 720-1600 kg/m3, has no plasticity and has water absorption ranging greatly from 2.0 to 30.0% or so. Most of bottom ash exhibits a grey color and may exhibit various colors such as yellow, black and pale ash colors depending on environment where it is generated. Unburnt carbon particles exhibit black colors, and the bottom ash containing silica and alumina components in a large amount exhibits pale ash and yellow colors.
Examples of a bottom ash treatment system include a direct sluicing system, a dewatering bin storing system, a water recirculation system, a chain conveyer system, etc.
The direct sluicing system is a system in which bottom ash discharged from a hopper disposed at the bottom of a boiler is directly transferred to an ash pond together with water through an ash transmission line. The direct sluicing process is performed such that bottom ash in the form of a clinker, which is mixed with water, is pulverized by a pulverizer mounted at an outlet of the hopper, and is then transferred to the ash pond with a jet pump. Such a direct sluicing system is very useful in case where the distance from a power plant to the ash pond is very short, and typically employs seawater since a large quantity of water is required for ash treatment. Further, a solution for preventing water pollution due to water used for ash treatment and reducing the quantity of water to be used employs a method of recirculating water used to the bottom ash hopper.
The dewatering bin storing system is a system in which bottom ash crushed is transferred to a dewatering bin together with water through an ash transmission line and is stored in the dewatering bin for 24 hours for the purpose of dewatering. Thereafter, the dewatered bottom ash is transported to an ash pond using a belt conveyor, a truck or the like so as to be landfilled in the ash pond or to be utilized for other purposes. Such a system is suitable for the case where the bottom ash is used as roadbed material or raw materials in cement manufacture.
The water recirculation system is a system in which bottom ash mixed with water is transferred to the dewatering bin to dewater the mixture in the same method as in the dewatering bin storing system. Particularly, it is unique that water discharged from the dewatering bin is transported to a settling pond or settling tank and is sedimented for recycling. This system is widely used in a large-capacity coal thermal power plant as it can minimize the amount of water to be used in the bottom ash treatment.
In addition, the chain conveyer system is a system in which a water-immersion type chain conveyor is mounted at a clinker hopper disposed at the bottom of the boiler so that bottom ash collected at the bottom of the boiler is rapidly cooled using water contained in the hopper, is pulverized and is then discharged to the outside of the hopper using the conveyor. Such a system is widely used in Europe centering around Germany since it permits low driving power and small installation space and requiring a small quantity of water to be used in bottom ash treatment.
This coal ash, a by-product generated from the coal thermal power plant was mainly treated in a coal ash disposal pond installed additionally in the power plant. In the past when the availability of the national land was high, the coal ash disposal was somewhat easy. However, recently, owing to the rapid increase of the demand for factory sites and the rise of the land values resulting from high economic growth rate, it has become difficult to find an ash pond requiring an area three to four times as large as the area for a power plant facility.
Thus, in case of fly ash, various research institutes and academic circles have conducted researches incessantly, and as a result, the fly ash is recycled as a variety of materials such as cement admixtures, embanking materials, earth reformation materials, lightweight aggregates, etc. Particularly, the fly ash occupies about 90% of the total recycling rate since the use of the fly ash as a concrete admixture and a raw material in cement manufacture is activated.
However, despite the above described various bottom ash disposal systems, most of the bottom ash is discarded and landfilled in an ash pond around the power plant or is landfilled in an inland area or coastal landfill, being mixed with the fly ash. Besides this, a small quantity of bottom ash is used as a roadbed embanking material around the power plant. Accordingly, this caused difficulties in securing ash disposal sites as well as environmental pollution problems. Thus, there is an urgent need for disposal of the bottom ash.
Moreover, since the physical and chemical property of the bottom ash itself is not good, difficulty is involved in easily recycling the bottom ash. As a result, review of a solution for recycling the bottom ash must be sufficiently performed. At the present day, advanced foreign countries has actively conducted review of applicability of the bottom ash as a roadbed aggregate, a fill-up material and a filler material so as to apply such bottom ash to the industrial field.
Examples of a conventional technique using the bottom ash as an aggregate include Korean Patent Laid-Open Publication No. 1997-074076 relating to a substitution of a part of a natural and artificial aggregate, Korean Patent Laid-Open Publication No. 1997-061815 entitled “a method of fabricating lightweight construction materials using bottom ash generated from combined heat and power plant” in which the bottom ash generated form the combined heat and power plant is partly used in the fabrication of the lightweight construction material, Korean Patent Laid-Open Publication No. 2002-0026794 entitled “concrete product using bottom ash and method for producing the same” in which the bottom ash is used as an aggregate to produce the concrete product, and Korean Patent Registration No. 10-047676 entitled “concrete composition using bottom ash as an aggregate substitute material mixed with concrete” in which the concrete composition contains cement using a fine aggregate of bottom ash substituting for sand and a natural coarse aggregate as cobblestone. However, the aforementioned conventional prior arts have a limitation in that in most cases, the bottom ash was combined with other natural aggregate to produce a concrete or lightweight construction material and its use amount is also restricted.
Coal ash can be used in a large amount in an aggregate and architectural field. Especially, in case of an aggregate as a construction material, the supply of an aggregate gradually becomes difficult due to the exhaustion of natural aggregate resources and occurrence of an environmental problem according to production of crushed stone. Potential amount of aggregate reserves is reduced in terms of the supply of domestic natural aggregate resources, and lots of problems with supply occur due to the remote location of aggregate production sites and a large increase in traffic congestion and distribution costs. Thus, it is expected that occupancy ratio of crushed stone to marine aggregate will increase in the future. Also, along with scale-up and quality enhancement of constructional structures, lightweightness of structural members becomes a critical factor, which causes a high-cost artificial lightweight aggregate to be manufactured using advanced materials of quartz or clay. Many researches have been conducted on preparation of concrete admixture and artificial lightweight aggregate using coal ash, paper mill sludge, slag, etc., in Korea. But the use of the artificial lightweight aggregate is relatively low as compared to foreign advanced countries due to a rise in production costs caused by high-temperature calcination, relatively low strength, difficulty of quality control and sluggishness of product development using produced aggregate.
In addition, as compared to the fly ash of existing coal ash, the bottom ash is nearly buried in ash pond, and hence large sized ash pond is required. This result in a dramatic construction cost and has an adverse effect on environment. There is therefore a need for an alternative bottom ash disposal method. However, since a method of recycling the bottom ash has several technical restrictions, the research on the bottom ash is not actively being conducted. Such a shortcoming is roughly divided into two types.
First, since particle size distribution is very heterogeneous, quality stability is remarkably deteriorated in the production of a lightweight aggregate and unburnt carbon needs to be separated in the course of production of bottom ash so as to maintain good physical properties. Also, since bottom ash particle having a particle size of more than 1 mm must be subjected to particle size separation or pulverization process, economic efficiency will decrease.
Second, since most of the thermal power plants adopt a method of storing the bottom ash together with water in an ash pond, the bottom ash contains a large quantity of moisture (10-30%), and removal of moisture is basically needed to recycle such bottom ash. This results in a restriction in an optimum moisture content range necessary for formation of lightweight aggregates, and hence there is a limitation in increasing the additional amount of the bottom ash. As a result, mass disposal of the bottom ash becomes difficult. In order to solve this problem, there is a need for complicated techniques in which a dry additional material is added to the bottom ash or the moisture content is reduced by drying the bottom ash.
In a conventional aggregate production field, researches have been actively performed on a method of recycling the fly ash which has been subjected to a disposal process. There have been numberous proposed techniques of producing an artificial aggregate using fly ash as in the following prior arts: Korean Patent Registration No. 10-150485 entitled “a method of producing artificial lightweight aggregate”, Korean Patent Registration No. 10-0150485 entitled “a method of producing high-strength artificial aggregate using fly ash as a main material”, Korean Patent Laid-Open Publication No. 10-1994-0025975 entitled “lightweight aggregate and preparation method thereof” and Korean Patent Registration No. 10-0056890 entitled “preparation method of artificial lightweight aggregate”.
However, an artificial aggregate using bottom ash is difficult to produce due to the aforementioned shortcomings.
Thus, the present inventors have confirmed that as a new construction material capable of replacing natural aggregates being exhausted, an economically efficient artificial lightweight aggregate containing bottom ash can be produced by disposing the bottom ash with a low recycling rate in a large amount, while making water absorption uniform to maintain physical properties through the maximum reduction of water absorption deviation of respective lightweight aggregates that may occur during mass production and have completed the present invention.